Don’t use it unless you must to achieve a certain effect. It has a gushy aura – the breathless excitement of a debutante commenting on an event that was exciting only to her…construct your sentences so that the order of the words will put the emphasis where you want it. Also resist using it to notify the reader that you are making a joke or being ironic. Readers are annoyed by your reminder that this was a comical moment. They are also robbed of the pleasure of making the discovery themselves. Humor is best achieved by understatement, and there’s nothing subtle about an exclamation point.